Secondary batteries such as lithium ion secondary batteries have characteristics such as compact size, light weight, high energy-density, and the ability to be repeatedly charged and discharged, and are used in a wide range of applications. A secondary battery generally includes battery members such as electrodes (a positive electrode and a negative electrode) and a separator that isolates the positive electrode and the negative electrode from one another and prevents short-circuiting between the positive and negative electrodes.
In recent years, battery members that have, as a protective layer, a porous membrane for increasing heat resistance and strength have been used.
Specifically, examples of porous membrane include those formed by bonding inorganic particles by a binder. Such a porous membrane is normally formed by preparing a slurry composition (hereinafter also referred to as “a composition for a porous membrane”) formed by dispersing inorganic particles and a binder in a dispersion medium such as water, applying the composition for a porous membrane on an electrode substrate formed by providing an electrode mixed material layer on a current collector or a separator substrate, and drying the composition.
Extensive improvements have been made on porous membranes in an aim to achieve higher performance of secondary batteries. For example, attempts have been made to improve a battery performance by changing properties and materials of inorganic particles in a porous membrane (see, for example, PTL1 and PTL2).
PTL 1 reports that, a porous membrane that has an optimal porosity, an excellent uniformity and a thermal stability can be formed by using, as inorganic particles, inorganic oxide particles such as α-alumina having specific properties.
PTL 2 discloses that metal hydroxide such as aluminum hydroxide that causes dehydration at a specific temperature is used as inorganic particles, and reports that a separator comprising a porous membrane that contains such metal hydroxide exhibits an excellent heat resistance, shutdown function, flame retardancy and handleability.